Mojtaba E. Kan, Hossein A. Taiebat and Mahdi Taiebat
In design of new embankment dams or evaluation of the performance of existing earthfill and rockfill dams, the Newmark-type Simplified Methods are widely used to estimate the earthquake-induced displacements. These methods are simple, inexpensive, and substantially less time consuming as compared to the complicated stress–deformation approaches. They are especially recommended by technical guidelines to be used as a screening tool, to identify embankments with marginal factor of safety. The methods would serve as a reliable screening tool had they always resulted in conservative estimates of settlements. However, a number of studies in the last 15 years show the contrary. This paper provides a critical review of the fundamental theory behind the simplified Newmark-type methods. Cases in which the results of the simplified methods are reportedly non conservative are further investigated and possible reasons are discussed, that may be taken into account in future design and investigations of Australian dams. The reliability of the simplified methods is examined based on the existing thresholds proposed in the literature and accounting for the embankment geometry and type, and for the seismic activity characterization. A recently proposed practical framework is further elaborated to demonstrate its effectiveness in the study of seismic behaviour of embankment dams. In particular, the case study of Zipingpu concrete faced rockfill dam in China is discussed where all widely used simplified procedures failed to predict the order of deformations experienced by the Dam under a recent strong earthquake event.
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Alberto Scuero, Giovanna Lilliu, Marco Scarella, Gabriella Vaschetti
Hardfill dams present technical and cost advantages. Placement is like in embankment dams, thus construction is fast. The typical trapezoidal shape makes possible use of local aggregates and low cement content. Despite the low strength material, these dams can be built on weak foundation, and resist earthquake and overtopping. However, being the material semi-pervious, they require an impervious facing. Until 2014 this was typically made with conventional concrete slabs with waterstops, or grout enriched hardfill. Concrete facings require heavy and costly equipment, long construction time, are expensive, frequently require maintenance.Construction of the facing can have a big impact on the overall construction costs of the dam. Replacing the concrete facing with a geomembrane lining is a cost-effective solution. This paper describes two hardfill dams’ projects with an exposed geomembrane as upstream liner: Filiatrinos (Greece, 2015), 55.6 m high,and Ambarau(Democratic Republic of the Congo, 2017), 19.30 m high.
Zivko R. Terzic, Mark C. Quigley, Francisco Lopez
The Mt Bold Dam, located in the Mt Lofty Ranges in South Australia, is a 54m high concrete arch-gravity dam that impounds Adelaide’s largest reservoir. The dam site is located less than 500m from a suspected surface rupture trace of the Willunga fault.Preliminary assessments indicate that Mt Bold Dam is likely to be the dam with the highest seismic hazard in Australia, with the Flinders Ranges-Mt Lofty region experiencing earthquakes of sufficient magnitude to generate shaking damage every 8-10 years on average. Prior evidence suggests that the Willunga Fault is likely capable of generating M 7-7.2 earthquakes.As part of the South Australia Water Corporation (SA Water) portfolio of dams, Mt Bold Dam is regularly reviewed against the up-to-date dam safety guidelines and standards. SA Water commissioned GHD to undertake detailed site-specific geophysics, geotechnical and geomorphological investigations, and a detailed site-specific Seismic Hazard Assessment (SHA) of the Mt Bold Dam area. The results of this investigation will be used to inform decisions related to planned upgrade works of the dam.Geomorphological mapping of Willunga Fault, detailed geological mapping, analysis of airborne lidar data, geophysical seismic refraction tomography and seismic reflection surveys,and paleoseismic trenching and luminescence dating of faulted sediments was conducted to obtain input parameters for the site-specific SHA.Discrete single-event surface rupture displacements were estimated at ~60 cm at dam-proximal sites. The mean long-term recurrence interval (~37,000 yrs) is exceeded by the quiescent period since the most recent earthquake (~71,000 yrs ago) suggesting long-term variations in rupture frequency and slip rates and/or that the fault is in the late stage of a seismic cycle. The length-averaged slip rate for the entire Willunga Fault is estimated at 38 ± 13 m / Myr. Shear wave velocity (Vs30) of the dam foundations was estimated based on geotechnical data and geological models developed from geophysical surveys and boreholes drilled through the dam and into the foundation rock. The nearest seismic refraction tomography (SRT) lines were correlated with the boreholes and those velocity values used in the Vs30 parameter determination. All relevant input parameters were included into seismic hazard analysis with comprehensive treatment of epistemic uncertainties using logic trees for all inputs.Deterministic Seismic Hazard Analysis (DSHA) confirmed that the controlling fault source for the Mt Bold Dam site is Willunga Fault, which is located very close to main dam site (420m to the West).For more frequent seismic events (1 in 150, 1 in 500 and 1 in 1,000 AEP), the probabilistic analysis indicates that the main seismic hazard on the dam originates from the area seismic sources (background seismicity).Based on deaggregation analysis from the site specific Probabilistic Seismic Hazard (PSHA), the earthquakes capable of generating level of ground motion for the 1 in 10,000 AEP event can be expected to occur at mean distances of approximately 22km from the dam site(with the mean expected magnitude atMt Bold Damsite estimated at Mw >6).For less frequent (larger) seismic events, the contribution of the Willunga Fault to the seismic hazard of Mt Bold Dam can be clearly noted with Mode distance in the 0-5 km range, which indicates that most of the seismic hazard events larger than the 1 in 10,000 AEP comes from the Willunga Fault. The Mode magnitudes of the events are expected to be Mode Magnitude at Mw= 6.6 for a segmented Willunga Fault scenario, and Mw= 7.2 for a non-segmented fault scenario.Consideration was also given to the upcoming update of the ANCOLD Guidelines for Earthquake, which calls for the determination of the Maximum Credible Earthquake (MCE) on known faults for the Safety Evaluation Earthquake (SEE) of “Extreme” consequence category dams. The MCE for Mt Bold Dam was estimated from the DSHA; in terms of acceleration amplitude, the MCE event approximately equals the 1 in 50,000AEP seismic events.
Zerui Lu, Behrooz Ghahreman-Nejad, Mahdi M. Disfani
Particle characterisation like size distribution and shape can greatly affect the mechanical behaviour of granular materials, and is closely related to the economics for engineering projects. For rockfill material in embankment dam construction, the particle size distribution (PSD) is fundamental to the design, quality control and numerical modelling. Traditionally, particle size distribution for engineering materials is obtained through physical sieving. However, with rockfill material, the size varies significantly and can range from gravels (+2mm) to cobbles (+60mm) and boulders (+200mm) with the maximum size usually limited to 1m, which makes the conventional sieving process considerably difficult to conduct as well as being time-consuming. Meanwhile, the advanced technology in computer image processing has created many possibilities in characterising particles within digital photographs, and therefore can be utilised as an effective alternative to the conventional sieve analysis. This method has been in use mainly in the mining industry over the past two decades to assist with rock fragmentation and process monitoring and control. Notwithstanding, the use of this technique in the dam industry for quality control of rockfill material has been rare. Thus, an innovative approach is proposed in this paper to estimate the PSD curves for rockfill material using image analysis along with the latest developments in aerial photography. The results of PSD analysis using the image processing software Split Desktop are presented and compared with the results from sieve analyses for verification. Recommendations are made to improve the process and increase the accuracy of the outcome. It is demonstrated that the proposed method has a reasonable accuracy and is a viable option for quality control in construction of rockfill structures such as rockfill embankment dams.
Dr Andy Hughes
On Hampstead Heath in Central London, just 3 kilometres from the centre of the city, there are more than 20 dams and reservoirs set within the landscape setting of Hampstead Heath. A number of dams were built in the 16th century and formed the original water supply to the City of London. They are set in a landscape laid out by the world renowned Humphrey Repton.Three of the embankments which are laid out in two chains of reservoirs across the Heath are subject to safety legislation in the UK. As such they were identified as being deficient in spillway capacity and thus fairly significant works were required to be carried out in this sensitive setting.The Heath is protected by the Hampstead Heath Act of 1870 which seeks to prevent significant changes to the Heath and thus it was quite clear that there would be opposition to any works on the Heath, even though they were required by law to protect persons and property downstream. In fact a significant lobby group formed which challenged the need for the works and also the legislation of the UK via a judicial review. This paper will describe the process by which significant stakeholder consultation was undertaken (costing more than £2M), the judicial review that took place in the Royal Courts of Justice, the option study and the major engineered elements carried out on the Heath.
Steven E Pells, Philip J N Pells
Junction reefs dam was designed in 1895 and constructed by 1897 as a multiple arch brick structure which was the first of its kind in Australia, and one of the earliest in the world. The dam was envisioned to provide mechanical and electrical power for gold mining. This paper provides an historical overview of the unique structure, and reassesses some of its engineering characteristics, such as the stress conditions in its unusual arches and reverse concrete gravity wing walls. The hydrology of the dam is re-assessed from the viewpoint of evaluating its potential as a mini hydro scheme. Commentary is also provided on the performance of its unlined spillway, which has been subject to regular spills for 120 years.